The latest issue of "Inspire", an al Qaeda magazine believed to be produced by the Yemen-based al Qaeda affiliate al Qaeda in the Arabian Peninsula (AQAP), featured a large advertisement for an upcoming article apparently written by al-Awlaki called "Targeting the Populations of Countries That Are at War With the Muslims". Al-Awlaki was a prominent member of AQAP.

The promotional ad uses as its background a picture of New York City's Grand Central Terminal, perhaps a clue, U.S. security officials said in a law enforcement bulletin, that it's "an allusion to the continued interest of extremists in general of targeting New York City for terrorist attacks."

While it is unclear if al-Awlaki managed to complete the piece before his death, it would hardly be the first time the New Mexico-born cleric has justified and urged individuals to launch terror attacks against U.S. civilian targets.

American officials said al-Awlaki's online sermons may have served as inspiration for more than a dozen terror plots against the homeland including the Fort Hood Massacre and the failed Christmas Day bombing.

Due to his ability to inspire so-called lone-wolf terrorists in the U.S., several U.S. officials have said al-Awlaki was more dangerous than even the late al Qaeda founder Osama bin Laden.

"If you look at his terrorist resume for the things he's been involved with, you can really call him the most dangerous man in the world," Lt. Kevin Yorke of the NYPD's Intelligence Division said in November 2010.

Earlier this year, America's chief counter-terrorism official Michael Leiter called him and AQAP "probably the most significant risk to the U.S. homeland."